Photodiodes are widely used in optical communications systems for the detection of optical signals. In particular, photodiodes are often included in optical transceivers and/or optical receivers for detecting the optical signals that are to be converted into electrical signals. The two most commonly used photodiodes for these applications include the p-material-intrinsic-layer-n-material (PIN) photodiode and the avalanche photodiode (APD).
While PIN photodiodes and APDs both ultimately accomplish the same result, namely the conversion of a received optical signal to an electrical signal, APDs use the avalanche phenomenon to produce an amplified electrical signal and thus are more suited for use in low, weak, or reduced light applications. This increased sensitivity, however, is achieved at the expense of having to provide a relatively high reverse bias voltage. For example, while PIN photodiodes often require reverse bias voltages between 5 and 20 V, APDs often require reverse bias voltages between 30 and 90 V.
The relatively high reverse bias voltage required for APDs is generated and controlled by a high voltage bias circuit. The high voltage bias circuit is essentially a DC-DC converter, which converts a low input voltage (e.g., less than about 5V) to a higher output voltage (e.g., from about 30V to 90V) using minimal current from the power generator.
FIG. 1 illustrates one example of a typical high voltage bias circuit. The high voltage bias circuit 100 includes a pulse-width modulated (PWM) chip 110, a field effect transistor (FET) switch Q1, an inductor L1, a diode D1, resistors R1, R2, and R3, capacitors C1 and C2, and a high voltage (HV) current monitor 120. When the FET Q1 is switched on, energy is stored in the inductor L1, while the diode D1 prevents the capacitor C1 from discharging to ground. When the FET Q1 is switched off, a voltage reversal is induced so that the energy stored in the inductor L1 is discharged, passes through the diode, and is transferred to the capacitor C1. As multiple cycles are completed, additional charge is built up on the capacitor C1 until a limited amount of current is supplied at the higher voltage VAPD. The circuit including Q1, L1, D1, and C1 is commonly referred to as a flyback circuit because the induced voltage reversal causes a “flyback” or “kickback” effect in the inductor L1. The PWM chip 110 is used to control the flyback action by providing a stream of pulses in which the pulse-width is modulated, thus maintaining a constant switching frequency. A feedback loop including resistor divider R2, R3 provides feedback needed to determine the duty cycle of the stream of pulses in order to control and regulate the output voltage VAPD. The HV current monitor 120 is provided to monitor the APD's average current, which is proportional to the optical signal strength. Typically, the current monitor 120 includes a sense resistor.
FIG. 2 illustrates another example of a high voltage bias circuit. The high voltage circuit 200 includes an external pulse generator 210, transistors Q1 and Q2, an inductor L1, diodes D1 and D2, capacitors C1 and C2, resistors R1, R2, R3, and R4, high voltage differential amplifier 220, and a current monitor 230. When the FET Q1 is switched on, energy is stored in the inductor L1, while the diode D1 prevents the capacitor C1 from discharging to ground. When the FET Q1 is switched off, a voltage reversal is induced so that the energy stored in the inductor L1 is discharged, passes through the diode D1, and is transferred to the capacitor C1. As multiple cycles are completed, additional charge is built up on the capacitor C1 and a limited amount of current is supplied at the higher voltage VAPD. The circuit including Q1, L1, D1, and C1 is commonly referred to as a flyback circuit because the induced voltage reversal results in the “flyback” or “kickback” effect provided by the inductor L1. The circuit including Q2, D2, R2, R3, R4, VREF, and the difference amplifier 220, serves as a shunt feedback loop. While the pulse generator 210 is used to induce the flyback effect by providing a stream of pulses, it is the shunt feedback loop that actually controls and regulates the output voltage VAPD. The HV current monitor 230 is provided to monitor the APD's average current, which is proportional to the optical signal strength. Typically, the current monitor 230 includes a sense resistor.
In practice, high voltage bias circuits, such as those shown in FIGS. 1 and 2, are external to the APD receiver and/or transceiver module that they control. For example, as shown in FIG. 3, a typical optical receiver 300 includes control circuitry 310 coupled to an APD receiver module 320. The APD receiver module 320 includes the APD 322, which converts the optical signal received from the optical input into an electric current, and an equalizing amplifier 326, which converts the electric current into a voltage. The control circuitry 310, which drives the APD 322, includes a high voltage bias circuit similar to that illustrated in either of FIGS. 1 and 2. For illustrative purposes, the high voltage circuit is shown broken down into two discrete parts, namely, a HV power supply and a current monitor. As shown in FIGS. 1, 2, and 3 it is preferred that the current monitor be mounted on the high side of the HV power supply. The HV current monitor is provided to monitor the APD's average current, which is proportional to the optical signal strength of an optical signal sensed at by the APD. The control circuitry 310 also includes a microprocessor coupled to the HV power supply. The microprocessor provides the reference voltage VREF and optionally provides corrections for the APD's temperature dependent response.
It is an object of the instant invention to provide a high voltage bias circuit for optical transceivers and/or receivers with a reduced number of components.
It is another object of the instant invention to provide a high voltage bias circuit for optical transceivers and/or receivers with increased resolution and/or increased power monitoring.